Modern land vehicles include a number of occupant safety devices including, for example, automatically tensioned seat belts, air bags, and rollover sensing systems. In the case of air bags, their deployment may be based on information from a variety of sensors such as speed sensors, accelerometers, and angle sensors. In some cases these sensors are located remotely from a processing module, but in other cases one or more of the sensors may be housed together with processors and actuation controls in a relatively small unit. When not mounted in or on a vehicle, these small units can, in general, be readily manipulated and held by a person.